The following two errors show up for Microsoft Office 2007 and 2003 users for a couple of reasons.
The key is incorrect. Verify that you have the correct key, and then retype it.
You have not entered a valid Product Key.
Please check the number located on the sticker on the back of the CD case or on your Certificate of Authenticity.
The first, and most obvious reason you might get this error is because you entered the product key incorrectly. Check the key you entered, and make sure it's correct. Some characters can look a lot alike. For example, zeros and the letter O look alike, eights and the letter B, and so on.
If you already have another version of Microsoft Office installed, you can get this error. You need to decide which version on Microsoft Office you want to use, and uninstall the version of Office that you aren't going to use.